Stephen, where did you get the graphic? It looks like something Bob would have, and it is wrong besides. At least it is wrong in the context you are trying to insinuate.

Without pulling out my psychrometric chart I will go out on a limb and say that graph represents the chart. But concrete RH measurements don't follow the standard chart. I don't know why, and greater minds are working on it.

Service temperature is the temperature the slab will be at during use. Slabs on grade don't vary much, suspended floors can vary quite a bit, especially if the above ceiling space is used as a return air plenum.

Have an odd slab where it is 2" ontop of a bitumin tar like hot mop over a 5-6" structural slab from the 1930's.

tester placed the wagner probes in the 1-1/2" so to not puncture the membrane / tar thing.

Results between 69% - 90% over 13 tests.

Here is my worry: slab temp > ambient temp. Somehow (and I've never seen this) the slab temp in the 50's and the air temp was 49. Ambient RH 57%.

I said I'd like to see readings taken when there is some drive with hotter air in the space (it has been cold in CA this winter). He more or less said hot air won't give good readings since this is the 'ideal' time for such testing with high humidity. I wouldn't call 57% ambient humidity high but...

Do you agree that these cold air temps encourage accurate RH readings in the Concrete?

I'm dragging this back. Now that it is the cold season I am getting RH tests that seem to be dropping and they seem to be more pronounced in cold buildings.

Are there any graphs showing: Concrete RH% vs Amient Air Temp or RH
Are there any graphs showing: Concrete RH% vs Concrete Temp

I know, I know, ASTM says blah blah blah, but I'm interested in the why. Basicly I am looking for how to say to someone WHY they have to have to be at service temp other than "it is ASTM". I want to be able to say
"well if you test with a colder room you will get a [lower/higher] concrete RH result"
or
"well if you test with a colder slab you will get a [lower/higher] concrete RH result"

Essentially I am worried that RH test results perhaps can be skewed just like CaCl by environmental factors but just in a different way.